The Earth's oceans are continually heated by the sun and cover nearly 70% of the Earth's surface. The temperature different between deep and shallow waters contains a vast amount of solar energy that can potentially be harnessed for human use. In fact, it is estimated that the thermal energy contained in the temperature difference between the warm ocean surface waters and deep cold waters within ±10° of the Equator represents a 3 tera-watt (3×1012 W) resource.
The total energy available is one or two orders of magnitude higher than other ocean-energy options such as wave power, but the small magnitude of the temperature difference makes energy extraction comparatively difficult and expensive, due to low thermal efficiency.
Ocean thermal energy conversion (“OTEC”) is a method for generating electricity which uses the temperature difference that exists between deep and shallow waters to run a heat engine. A heat engine is a thermodynamic device placed between a high temperature reservoir and a low temperature reservoir. As heat flows from one reservoir to the other, the engine converts some of the heat to work. This principle is used in steam turbines and internal combustion engines. Rather than using heat energy from the burning of fuel, OTEC power draws on temperature differences caused by the sun's warming of the ocean surface.
One heat cycle suitable for OTEC is the Rankine cycle, which uses a low-pressure turbine. Systems may be either closed-cycle or open-cycle. Closed-cycle systems use a fluid with a low boiling point, such as ammonia, to rotate the turbine to generate electricity. Warm surface seawater is pumped through a heat exchanger where the low-boiling-point fluid is vaporized. The expanding vapor turns the turbo-generator. Then, cold, deep seawater—pumped through a second heat exchanger—condenses the vapor back into a liquid, which is then recycled through the system. Open-cycle engines use the water heat source as the working fluid.
As with any heat engine, the greatest efficiency and power is produced with the largest temperature difference. This temperature difference generally increases with decreasing latitude (i.e., near the equator, in the tropics). But evaporation prevents the surface temperature from exceeding 27° C. Also, the subsurface water rarely falls below 5° C. Historically, the main technical challenge of OTEC was to generate significant amounts of power, efficiently, from this very small temperature ratio. But changes in the efficiency of modern heat exchanger designs enables performance approaching the theoretical maximum efficiency.
OTEC systems have been shown to be technically viable, but the high capital cost of these systems has thwarted commercialization. Heat exchangers are the second largest contributor to OTEC plant capital cost (the largest is the cost of the offshore moored vessel or platform). The optimization of the enormous heat exchangers that are required for an OTEC plant is therefore of great importance and can have a major impact on the economic viability of OTEC technology.
There are many existing heat-exchanger designs that can be considered for use in an OTEC system. But as the following discussion shows, there are, as a practical matter, no good choices.
Conventional “shell and tube” heat exchangers are widely available for marine use. But the overall heat transfer coefficient, U, that is associated with reasonable pressure drops for OTEC is typically below 2000 W/m2K. This drives the size and cost for this type of heat exchanger too high for economic consideration.
Compact heat exchangers/plate-frame designs, which comprise many narrowly separated plate pairs, are extensively used in the chemical and pharmaceutical industries. The U value for plate-frame designs may approach 2300-2500 W/m2K. In order to achieve this level of heat transfer in an OTEC system, however, high pumping power is necessary to force seawater through the heat exchanger to overcome a pressure drop that can exceed 5 psi (3.5 m head loss). Further, the upper value of the heat transfer coefficient is restricted by the use of materials such as high-alloy steel or titanium (which have relatively poor thermal conductivity but mitigate the effects caused by exposure to corrosive materials, such as chlorides, etc.), by the minimum plate thickness needed for stamped plate design, and by the relatively low water flow velocities required to maintain an acceptable pressure drop.
OTEC-optimized tube designs also fall into the compact heat exchanger category. These include Vertical Fluted Tube and Folded Tube heat exchangers. Both have somewhat higher U values than plate-frame designs: typically in the range of about 2700-3400 W/m2K. But a substantial pressure drop and potential for crevice corrosion in regions of stagnant flow are a concern for the optimized tube designs.
Brazed aluminum-fin heat exchangers are used throughout the cryogenics industry. These heat exchangers see large scale marine use in Liquid Natural Gas (LNG) re-gasification facilities. Brazed aluminum-fin heat exchangers were developed and tested for OTEC use in the 1980s at Argonne National Labs (ANL). See, U.S. Pat. No. 4,276,927 (“Plate type heat exchanger”) and U.S. Pat. No. 4,478,277 (“Heat exchanger having uniform surface temperature and improved structural strength”).
One of the main technical challenges addressed in these patents was the segregation of braze joints away from seawater passages to reduce the potential for corrosion. An optimized folded-fin design was used to minimize boundary layer resistance in boiling/condensing working fluid.
Through the mid 80's to the early 90s, various aluminum heat exchanger modules and alloys were tested in an actual OTEC environment. These instrumented and remotely-monitored tests correlated heat transfer performance and seawater chemical and physical properties with corrosion in the heat exchangers. As a result of this extended testing, it was concluded that several relatively inexpensive aluminum alloys should survive well in an OTEC application.
The form factor for the heat exchangers being tested was mostly shell and tube type. It was concluded that fabrication, out of aluminum, of shell-and-tube heat exchangers of sufficient surface area would be prohibitively expensive. “Roll bond” heat exchanger panels were proposed as an alternative, which provide the larger surface areas required for OTEC applications at roughly twenty percent the cost of equivalent shell and tube units.
In 1989, roll-bond panels were inserted into some the heat exchangers that were being tested in the OTEC environment. This testing led to the development of roll-bond type heat exchanger panels that were actually installed in a 50 kW plant built in 1996. During the first year of testing, serious ammonia leaks were experienced due to corrosion. The corrosion was due to electrolysis, which was caused by the spacer material between the aluminum panels.
The heat exchangers were re-manufactured and, after some difficulties with brazing associated with the inlets/outlets, the plant was reassembled and additional performance and corrosion data were collected. Based on these results, additional roll-bond modules were fabricated and tested in a simulated OTEC environment at a power plant in England.
By the mid-1990s, government funding of OTEC had concluded. Remaining hurdles for compact aluminum heat exchangers at that time included concerns over the placement of brazed sections within a heat exchanger core.
With today's growing need for energy, using a renewable constant source is a desirable solution. As a consequence, there is a renewed interest in OTEC power plants. But development of an OTEC heat exchanger that accommodates high flow rates while minimizing pumping parasitic losses and offering long life in the ocean environment remains elusive.